The present invention relates to the art of magnetic resonance imaging (MRI). It finds particular application in conjunction with magnetic resonance angiography (MRA), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Commonly, in MRI, a substantially uniform, temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged is placed. Via magnetic resonance radio frequency (RF) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped (via RF pulses) into a plane transverse to the main magnetic field such that they precess or resonate. In turn, the resonating dipoles are allowed to decay or realign with the main magnetic field thereby inducing magnetic resonance echoes. The various echoes making up the MRI signal are encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI apparatus is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data lines position in k-space is determined by its gradient encoding. Ultimately, employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space data.
With regard to MRA, typically, a bolus of contrast solution is introduced into the subjects vascular system which aids in the visualization of the blood vessels and/or other associated anatomy being imaged. Generally, it is desirable to time any image acquisitions such that they commence at the arrival of the bolus in the region or volume of interest. Moreover, it is desirable to achieve a temporal resolution significantly fine enough to accurately track the bolus within the region or volume of interest.
With respect to the timing of an acquisition, one previously developed technique involves the attending medical personnel or technician simply estimating the time from introduction to arrival based upon the available information on the vascular system and/or blood flow rates. There are no direct measurements or observations made to determine the actual arrival of the bolus in the region or volume of interest. Rather, the acquisition simply begins at the expiration of the estimated time period. Due to uncontrollable factors that varied from time to time and patient to patient (e.g., the variable blood flow rates, the distinct vascular systems of different patients, etc.), often the estimates are inexact. Consequently, this technique tends to be less precise than desirable.
In another previously developed acquisition timing technique, a direct measure or observation is made to determine the arrival of the bolus. In this technique, prior to arrival, quick low resolution scans are focused on the blood vessel which would carry the bolus into the region or volume of interest. Detection of a relatively larger signal indicates that the bolus has entered the region or volume of interest. In turn, detection of the jump in signal strength triggers the acquisition. However, as the bolus has already arrived, acquisition is started later than is otherwise desired. That is to say, the entrance of the bolus into the region or volume of interest is not acquired. Rather, there is some delay between detection of the bolus and acquisition.
The present invention contemplates a new and improved technique for addressing temporal resolution and acquisition timing which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging is provided. The method includes subjecting a region of a patient being imaged to magnetic resonance imaging pulse sequences thereby generating magnetic resonance echoes emanating from the region. The echoes are encoded with magnetic gradients and collected as k-space data in separate consecutive acquisitions. Each acquisition successively fills in one of a number of distinct subsets of k-space based on the encoding of the collected echoes for that acquisition. In turn, a reconstruction algorithm is applied to the k-space data after each acquisition to generate a series of temporally updated image representations of the region.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging includes generating magnetic resonance echoes which emanate from a region of a patient being imaged. The echoes are acquired as sampled data with each acquisition resulting in a distinct fractional portion of k-space being filled in. The echoes are repeatedly generated and acquired until k-space is completely filled in such that the distinct fractional portions of k-space are interleaved with one another. In turn, a reconstruction algorithm is applied to the data in k-space to generate an image representation of the region.
In accordance with another aspect of the present invention, a magnetic resonance imaging apparatus is provided. It includes a main magnet that generates a substantially uniform, temporally constant main magnetic field through an examination region of a subject being imaged. A magnetic gradient generator produces magnetic gradients in the main magnetic field across the examination region. A transmission system includes an RF transmitter that drives an RF coil which is proximate to the examination region. A sequence control manipulates the magnetic gradient generator and the transmission system to produce a plurality MRI pulse sequences. Each of the MRI pulse sequences induces magnetic resonance echoes which emanate from the examination region. A reception system includes a receiver that receives and samples the echoes to acquire k-space data such that distinct interleaved subsets of k-space are successively filled in and updated with each acquisition. A data memory which stores the k-space data from the receiver. A reconstruction processor that accesses the data memory following each successive acquisition to reconstruct the k-space data into temporally resolved image representations of the examination region, and an output device converts the image representations into a human viewable display.
One advantage of the present invention is improved temporal resolution.
Another advantage of the present invention is that in MRA image acquisition is readily timed with the arrival of a contrast bolus.
Yet another advantage of the present invention is that relatively long acquisitions can be realized without subjecting a patient to an uncomfortably long breath-hold time.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.